Tanning with glutaraldehyde



United States Patent TANNING WITH GLUTARALDEHYDE Martin L. Fein and Edward M. Filachione, Philadelphia,

Pa, assignors to the United States of America as represented by the Secretary of Agriculture No Drawing.

A non-exclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with power to great sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This application is a continuation-in-part of application Serial No. 527,577, filed August 4, 1955, now abandoned.

This invention relates to the tanning of untanned animal skins and has among its objects the provision of improved tanning processes utilizing a new tanning agent.

The tanning of hides is an ancient art and a great array of agents and processes have been used or proposed for its accomplishment. Tannage stabilizes the hide against dimensioned changes, microbiological deterioration and other deleterious effects. A characteristic effect shown by most tanning agents and commonly used to follow the course of tanning processes is the elevation of the shrink temperature. Green cattle hide shrinks irreversibly when heated above about 65 C. in the presence of water. During a tanning process the shrink temperature of the hide is raised to a point characteristic of the particular tanning agent and process and of the type hide used. In all conventional tannages this shrinkage is irreversible. For many applications, such as those involving exposure to steam or hot water, it is a great disadvantage to have the leather undergo irreversible shrinkage.

Another great disadvantage inherent in most conventional vegetable tannin-g processes is the inordinately long time required. The great bulk of commercial heavy leather is tanned with vegetable tannins by a process requiring several weeks. Also, vegetable tannins are becoming more and more scarce and expensive. In addition, our supply is uncertain in times of emergency, since most of it is imported.

Various synthetic materials have been used as tanning agents, formaldehyde being among them. Objections to use of formaldehyde include toxicity, irritating odor, and deficiences in various properties of the leather produced.

Several other aldehydes have been tested as tanning agents. Acetaldehyde shows only feeble action while its higher homologs are inert. Glyoxal and pyruvaldehyde are fairly etfective but are either not readily available or yield leather deficient in many properties. It has been generally assumed that thehigher homologous dialdehydes are progressively less elfeotive than glyoxyl, just as' the higher homologous monoaldehydes are less effective than formaldehyde and, in general, this is true.

We have now discovered that, contrary to all expectations, glutaraldehyde is an excellent tanning agent, has a very rapid tanning action over an extremely wide pH range, that is, either acidic, neutral or alkaline solutions may be employed, yields leather of excellent appearance, flexibility and stability, and most surprisingly, the leather so tanned not only has the unique property of reversible with a valuable plumping action.

shrinkage, but is remarkably stable to action of perspiration. By reversible shrinkage we mean that the leather, after being heated above its shrinkage temperature in the presence of Water causing the leather to shrink, expands and essentially regains its original dimensions when 'cooled. This cycle of shrinkage and expansion may be repeated Without any apparent change in the leather.

We have further found, much to our surprise, that the leather tanned with glutaraldehyde is unique with regard to the stability of the leather so tanned. Stability of leather, for example, resistance to deterioration by perspiration, can be measured by a method commonly used by those skilled in the art (Colin-Russ J.I.S.L.T.C., 3'1, 329 (1947)). In this method the leather is exposed to a synthetic perspirant made up of urea, sodium lactate, disodium phosphate, sodium chloride, and water buffered to pH of 7.0 to 7.1. The shrinkage caused by this synthetic perspirant is a measure of the stability or resistance of the leather toward perspiration, that is the greater the shrinkage produced in this test, the less stable the leather. Quite unexpectedly the glutaraldehyde tanned leadthers were stable to this test in contrast to other aldehydes for example succinaldehyde, glyoxal, and formaldehyde. In fact, glutaraldehyde was superior to conventional tanning materials such as chrome or vegetable tannins, in imparting this desirable property to the letter. The resistance to perspiration of the glutaraldehyde' tannage was not impaired even though the conditions of tanning varied over a Wide pH range, for example 4.5 to 10.

We have further found that glutaraldehyde tannage can be combined with mineral tannages such as chrome, aluminum, and zirconium or with a vegetable tannage such as quebracho, sulfited quebracho, wattle and chestnut. The combination tannage can be applied directly in one operation or as separate operations.

According to the invention, untanned animal skins are impregnated with glutaraldehyde as the tanning agent, dissolved in an inert solvent, until the animal skin is tanned. Preferably, the tanning is carried out in aqueous solution and at a pH in the range of about 2 to 10, that is, in the acid range of from about 2 up to 7, under neutral conditions of pH 7, and in the alkaline range from 7 to about 10.

Glutaraldehyde tannage may be conducted in an inert solventwhich may be either organic or aqueous solution at neutral, alkaline or acid pH. By operating in aqueous acid solutions a rapid tannage is obtained simultaneously An aqueous alkaline solution tannage is somewhat more rapid and utilization of glut-araldehyde is more quantitative. Rapid tannage is also obtained in a suitable inert organic solvent, such as a lower aliphatic ketone, for example, acetone, but sucha process involves higher costs in the handling and recovery of the solvent.

We prefer to use glutaraldehyde itself in our process, however, derivatives which can be hydrolyzed to glut-araldehyde such as glutaraldehyde acetals, may be used in acid solutions. For example, glutaraldehyde acetals, 1,4-pyran, and 2-alkoxy-3,4-dihydro-2H-pyrans such as 2-ethoxy-3,4-dihydro-2H-pyran all of which are hydrolyzable to glutaraldehyde, may be used in place of glutaraldehyde.

case for example pickled skins may be treated with the 2-alkoxy dihydropyran whereby the acid from the pickled skins effects hydrolysis of the dihydropyran. After hydrolysis the pH may be adjusted to any desired value and tanning continued.

The following Examples 1-28 are illustrations of the invention. in each of the Examples 1-20 (Table 1), two strips (1" x 3" approximately) of acetone dehydrated 7 These may be first hydrolyzed or the hydrolysis can be accomplished in situ. In the latter hide (brought to the isoelectric point before dehydration) were used. Shortly before chemical treatment with glutaraldehyde, and with succinaldehyde and glyoxal for comparative purposes, was started the strips were soaked in cold water until completely wet. For each example two wet strips were placed in a 4-ouncwide mouth bottle (fitted with a screw cap) containing the appropriate chemical mixture as outlined. The pH of the chemical mixture was recorded when a sample was cut from the strips at given time intervals, usually 1 hour, 3 hours, 6 hours, 1 day, 2 days, 3 days, and 4 days. The samples /2" x 2%") were cut from the original 1" x 3" strips for the purpose of determining shrinkage temperature (Ts) in water. The shrinkage temperatures were determined by a procedure giving results essentially equivalent to those attained by the standard procedure of the American Leather Chemists Association. Each cut sample x 2%") was soaked in cold running water from 1 to 4 days, to assure complete washing, before the shrink temperatures (Ts) were determined. In no case did Ts change significantly after the third day of chemical treatment. However, in the case of the glutaraldehyde treatment, maximum Ts was attained in 3 to 6 hours for cowhide and less than 1 hour for calfskin.

In all the experiments summarized in Table -I, the aldehydes were added as aqueous solutions in amounts in excess of that which the hide would absorb. Each sample of hide was immersed in 50 ml. of water or of the aqueous solution of the other reagent specified, the aldehyde was added, and the bottle and contents were rotated continuously at room temperature for the times shown.

1y, glutaraldehyde was still more effective than glyoxal (Examples 10 and 13). Thus, glutaraldehyde was more efiective than either of the controls over the entire range of pH. Furthermore, analyses of spent tanning liquors showed that, especially under alkaline tanning conditions, glutaraldehyde is utilized more quantitatively than the other dialdehydes. That is, during tanning the skins will take up 95% or more of the glutaraldehyde in a tanning solution. Hence, upon experimental determination of the glutaraldehyde required to tan a given weight of skin, only this calculated amount need be incorporated into the tanning solution, whereas considerable excesses of the control dialdehydes were necessary to insure adequate tannage in their respective solutions, and then over a longer period of time.

Glutaraldehyde was also found to be significantly superior to its higher homolog, 3-methyl glutaraldehyde, again emphasizing the unobviousness of glutaraldehyde as a tanning agent. A still higher dialdehyde, whydroxyadipaldehyde, was tested in a series of experiments similar to those of Examples 1-20 but was found to have only very feeble tanning action, inferior even to glyoxal.

In addition to the pH range disclosed in Table I, similar glutaraldehyde tannages were conducted successfully in the pH range of 1.8 to 3.0.

Several experiments were run to determine the efiect of vegetable retannage of aldhyde tanned cowhide and calfskin. The materials were tanned as in Examples 10 and 18 and had shrink temperatures of 85 C. and 82 C., respectively. They were then placed in a 10% aqueous solution of vegetable tannins /3 sulfited quebracho, /3 chestnut) and allowed to soak with oc- TABLE I.-TANNING WITH DIALDEHYDES COWHIDE Ts, 0., After Various Tanning Periods Ex. pH

No. Aldehyde Used Other Reagent Range Hour Hours Hours Hours Hours 1 Glutaraldehyde- 3. 3-4. 2 81 84 85 2 Succinaldehyde. 3. 2-3.3 68 72 72 77 77 3.... Glyoxal- 3.0-3.3 63 63 63 69 70 4- Glutaraldehyde- 3. 8-4. 7 77 82 82 83 83 5 Succinaldehydo. 3. 5-4.0 72 78 79 8O 80 6 Glyoxal 3. 9-4.4 65 67 68 78 7- Gluteraldehyda 4. 2-5. 0 75 83 86 86 86 8 Succinaldehyde 3. 6-4. 1 72 77 77 S1 9 Glyoxal 3. 7-4.1 64 66 68 76 83 10- Glutaraldehyde. 8. 8-7.8 75 82 84 84 84 11 Succinaldehyde 6. 9-7. 4 68 71 71 73 75 12- Glyoxal 8.1-7. 3 71 77 78 77 81 13-.. Glutaraldehyde. 8.5-7.9 79 79 82 83 83 14- Succinaldehyde- 7. 3-7. 8 69 70 70 74 76 15 G oxal 7. 5-7.2 68 77 78 83 83 16 Glutaraldehyde- 9. 8-9. 4 78 83 84 86 86 17- Succinaldehyde. 9.1-8.8 68 75 75 75 78 CALFSKIN 18"- Glutaraldehyde. NagSO4, 5%; NaHOO 2% 7.6-8.0 83 88 88 83 83 19"- Succinaldehyde- N 828 O4, 5%; NaHOO 2% 7. 4-7. 9 71 72 72 74 77 20 Glyoxal NagsOi, 6%; NaHCO 2% 7.6 7.4 78 77 79 83 VEGETABLE TANNED HIDE 21 Glutara1dehyde 2. 8-3. 1 85 88 90 93 93 22--- Succinaldehyde. 2.9-2. 7 83 85 85 92 93 23. Glyoxal 2. 8-2. 9 84 84 85 87 88 l\ About 10 g. of cowhide or 3 g. of calfskin (on dry weight basis) were treated with approximately ml. of 0.5 in. aqueous dialdehyde solution containing indicated amounts of butler and/or salts.

casional agitation. After two days the calfskin was thoroughly tanned and its Ts had been raised to 93. The cowhide, which was quite heavy, required a longer time for complete penetration, its Ts then being 92". The tannins penetrated it evenly from both sides, there being no evidence of case hardening.

While we have used cowhide and calfskin in Examples 1-20 described above because of their commercial imhyde (see Examples 11-12 and 14-15), but, surprising- 75 portance, the process of the invention is not limited thereto. In fact, any animal skin that is commonly used in the preparation of leather may be tanned with similar good results by use of our process; for example, pigskin, sheepskin, goatskin and horsehide. By the term animal skins we mean to include all such hides and skins.

Having shown the versatile nature of our invention the following examples further illustrate its practical application. The scope of our invention, however, is not limited to the specific conditions defined in these exam- ,ples. The amount of reagents used is given not only in grams but, parenthetically, also as the percentage or parts per hundred based on the drained pickled weight of the skin. This latter method is commonly used in the leather industry to express amounts of material used.

Example 24 A pickled and degreased Sudan sheepskin was cut along the backbone into two matched halves. One half weighted 600 grams and was placed in a solution comprising 1200 grams (200% based on drained pickled weight of skin) of water, 66 g. (11%) sodium chloride, and 24 grams (4%) of anhydrous sodium acetate. The skin was drummed in this solution for 2 hours, the pH at the end of this .time was 4.6 and Ts of the untanned sheepskin was 52 C. Next 72 grams of a 25% aqueous solution of glutaraldehyde was added in two feeds of 36 grams each 2.5 hours apart. This corresponds to a total of 18 grams of pure aldehyde or 0.03 mole glutaralde hyde per 100 g. of pickled skin. The skin was drummed in the tanning solutions for a total of 24 hours, the pH remaining substantially constant at 4.4 to 4.6. The tanned skin was washed, set out and dried at room temperature.

The leather was soft and pliable even though it contained no fat liquor. The shrinkage temperature of the leather was 82 C.

The stability of the glutaraldehyde tannage was meas ured by determining the resistance of this leather to a synthetic perspirant according to the method of Colin- Russ. A specimen of this leather when submitted to the perspiration test showed no shrinkage in area. Furthermore, the leather remained flexible after this test.

Substantially the same results were obtained when the glutaraldehyde tannage was applied to other skins or under other conditions of tanning.

Example 25 The matching half of the Sudan sheepskin mentioned in Example 24 was tanned with succinaldehyde by a procedure identical to the tanning of its mate with glutaraldehyde. The skin, weighing 560 grams, was placed in a solution comprising 1120 g. (200% based on the drained pickled weight of the skin) of water, 62 g. (11%) of sodium chloride, and 22.4 g. (4%) of anhydrous sodium acetate. The skin was drummed in' this solution for 2 hours, the pH at the end of this time was 4.6 and the Ts of the untanned skin was 520 C. Next 52.6 g. of a 27.5% aqueous solution of succinaldehyde was added in two feeds of 26.3 g. each 2.5 hours apart. This corresponds to a total of 14.5 g. of pure aldehyde or 0.03 mole of succinaldehyde per 100 g. of pickled skin. The skin was drummed in the tanning solution for a total of 24 hours, the pH remaining substantially constant at 4.4 to 4.6. The tanned skin was washed, set out and dried at room temperature. The shrinkage temperature of the leather was 77 C.

This leather was submitted to the Colin-Russ perspiration test as Was done in Example 24 showed a shrinkage of 29% in area, reflecting a marked instability of the succinaldehyde tannage. Furthermore, the leather became quite stiff as a result of the test.

This result is in marked contrast to the results obtained with the glutaraldehyde tanned leather (Example 24).

In fact the glutaraldehyde tannage is unique in its property of resisting perspiration. Glyoxal and formaldehyde tanned sheepskins when submitted to this test underwent area shrinkage of about 50 and 60% respectively and became very hard and useless as regards leather properties.

Example 26 Three pickled cabretta skins were tanned as follows: The drained pickled sheepskins, weighing 2,115 grams, were added to a solution comprising 2,115 grams (100% of the drained pickled weight of the skin) of water, 127 grams (6%) of sodium chloride, and 254 grams (12%) of commercial glutaraldehyde solution containing 25% by weight of glutaraldehyde. The skins were drummed in this solution for V2 hour, the pH being 2.3 at the end of this time. Then 85 grams (4%) of anhydrous sodium formate was added and the drumming continued for 2 hours. The pH at the end of this time was 4.0 and Ts of the skin was 70 C. Next 63.5 grams (3%) of sodium bicarbonate was added and drumming continued for 1% hours. The pH was 6.6. This was followed by an additional 10.5 grams (0.5%) of sodium bicarbonate and drumming continued for 2 hours. The pH at the end of this sequence of steps was 7.0 and the Ts of the skins was raised to 82 C. To the drum was then added 37 grams (1.75%) of 90% formic acid solution. After drumming for /2 hour the pH was 4.4. The leather (Ts 83 C.) was washed and drained.

These skins tanned with glutaraldehyde were fat liquored and finished by the conventional post tanning steps applied to chrome tanned sheepskin garment leather. The finished leather was soft and mellow, resisted deterioration by perspiration, and was a most satisfactory example of garment type leather.

Example 27 A pickled degreased Syrian sheepskin of commerce weighing 968 g. was added to a solution made of 968 g. (100%) of water, 97 g. (10%) of anhydrous sodium sulfate and 116 g. 12%) of aqueous glutaraldehyde containing 25% by weight of glutaraldehyde. After drumming for /2 hour the pH of the tanning solution was 2.1. Then to the drum was added simultaneously 48 g. (5%) of sodium bicarbonate and 29 g. (3%) of magnesium oxide. The addition of these two alkaline agents brought the pH of the tanning solution to above 9 and drumming was continued for 24 hours. At the end of this time the pH was 9.2 and Ts of the leather was 83 C. Analysis of the tanning liquor showed that over of the glutaraldehyde was consumed during tanning.

The glutaraldehyde tanned skin was acidified to a pH of about 5 with acetic acid and processed into finished garment leather. This leather showed substantially the same properties, i.e., softness and perspiration resistance, as that tanned in Example 26.

Example 28 A gram portion of pickled degreased Turkish sheepskin was added to a solution prepared from 100 g. (100% based on the drained pickled weight) of water, 6 g. (6%) of sodium chloride, and 4 g. (4%) of 2- ethoxy-3,4-dihydro-2H-pyran. This quantity of the sub stituted dihydropyran corresponds to about 3 g. of pure glutaraldehyde. The pickled skin in this solution was tumbled for 3 /2 hours, and the pH of the solution at the end of this time was 2.0. Then 5 g. (5 of anhydrous sodium acetate was added and tumbling continued for 24 hours. At the end of tanning the pH was 4.1 and Ts was 81 C. The leather was washed and allowed to air dry and was fully equivalent to that obtained when glutaraldehyde itself was used for tanning.

We claim:

1. The process of tanning an untanned animal skin comprising treating the animal skin with glutaraldehyde,

as the tanning agent, dissolved in an inert solvent selected from the group consisting of water and a lower aliphatic ketone, at a pH in the range of about from 2 to 10, until 'the animal skin is tanned.

2. The process of claim 1 wherein the inert solvent is water. i

3. The process of tanning an. untanned animal skin comprising immersing an acid pickled skin in an aqueous solution of a compound selected from. the group consisting of a glutaraldehyde acetal, 1,4-pyran, and an 2- alkoxy-3,4-dihydro-2H-pyran, whereby glutaraldehyde is formed in situ, adjusting the pH of said aqueous solution to a value in the range of about from 2 to 10, and tanning said skin in the solution of adjusted pH until the skin is tanned.

ReEerences Cited in the file of this patent UNITED STATES PATENTS 2,516,284 Winheim July 25, 1950 

1. THE PROCESS OF TANNING AN UNTANNED ANIMAL SKIN COMPRISING TREATING THE ANIMAL SKIN WITH GLUTARALDEHYDE, AS THE TANNING AGENT, DISSOLVED IN AN INERT SOLVENT SELECTED FROM THE GROUP CONSISTING OF WATER AND A LOWER ALIPHATIC KETONE, AT A PH OF THE RANGE OF ABOUT FROM 2 TO 10, UNTIL THE ANIMAL SKIN IS TANNED. 